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    SubjectRe: [PATCH] mm: disable preemption in apply_to_pte_range
    On Fri, 2009-02-13 at 09:24 -0800, Jeremy Fitzhardinge wrote:
    > Peter Zijlstra wrote:
    > >> The specific rules are that
    > >> arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() require you to be
    > >> holding the appropriate pte locks for the ptes you're updating, so
    > >> preemption is naturally disabled in that case.
    > >>
    > >
    > > Right, except on -rt where the pte lock is a mutex.
    > >
    > Hm, that's interesting. The requirement isn't really "no preemption",
    > its "must not migrate to another cpu". Is there a better way to express
    > that?

    Not really, in the past something like migrate_disable() has been
    proposed, however that's problematic in that it can generate latencies
    that are _very_ hard to track down, so we've always resisted that and
    found other ways.

    > >> This all goes a bit strange with init_mm's non-requirement for taking
    > >> pte locks. The caller has to arrange for some kind of serialization on
    > >> updating the range in question, and that could be a mutex. Explicitly
    > >> disabling preemption in enter_lazy_mmu_mode would make sense for this
    > >> case, but it would be redundant for the common case of batched updates
    > >> to usermode ptes.
    > >>
    > >
    > > I really utterly hate how you just plonk preempt_disable() in there
    > > unconditionally and without very clear comments on how and why.
    > >
    > Well, there's the commit comment. They're important, right? That's why
    > we spend time writing good commit comments? So they get read? ;)

    Andrew taught me that indeed, but still when looking at the code its
    good to have some text there explaining things too.

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      Last update: 2009-02-14 10:59    [W:0.026 / U:4.852 seconds]
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